[Mono.Android] add RequireViewById<T>() for better NRT support (#7158)

Context: https://cs.android.com/android/platform/superproject/+/master:frameworks/base/core/java/android/view/View.java;l=25322
Context: https://developer.android.com/reference/android/view/View#requireViewById(int)

In porting Xamarin.Android samples to .NET 6, using latest C# 10
features like `$(Nullable)=enable`, you run into a pattern such as:

	var foo = FindViewById<TextView>(Resource.Id.foo);
	ArgumentNullException.ThrowIfNull(foo);
	foo.Text = "bar";

`var` here is a nullable `TextView?`, so you have to `null`-check.
This is pretty verbose, and actually seems to make nullable reference
types annoying…

API-28 introduced a [`View.requireViewById(int)` method][0]:

	// Java
	/* partial */ class View {
	    <T> public final T requireViewById (int id);
	}

This was bound as as a non-generic [`View.RequireViewById(int)`][1]
method:

	// C#
	partial class View {
	    public Java.Lang.Object RequireViewById (int id);
	}

"Overload" `View.RequireViewById()` with a generic version, similar
to the existing [`View.FindViewById<T>(int)` overload][2]:

	// C#
	partial class View {
	    public T RequireViewById<T>(int id)
	      where T : View
	    { … }
	}

Introduce an equivalent `Activity.RequireViewById<T>(int)` method:

	// C#
	partial class Activity {
	    public T RequireViewById<T>(int id)
	      where T : View
	    { … }
	}

This allows developers to simply:

	TextView foo = RequireViewById<TextView>(Resource.Id.foo);
	foo.Text = "bar";

or use `var` if you prefer.

If the `id` isn't found, a `Java.Lang.IllegalArgumentException` is
thrown with a message containing the `typeof(T)` and hex value of id.

Note: these new `RequireViewById<T>()` methods are only available on:

  * .NET 7 (xamarin-android/main)
  * .NET 6 + API-33 (`net6.0-android33.0`)
  * Future API-34 bindings.

Meaning that "Classic" Xamarin.Android 13.0 will *not* contain these
methods.  It's too late to easily change
`$(TargetFrameworkVersion)`=v13.0, and if we added it we could get
into a state where a future NuGet package used these methods, was
built against Xamarin.Android 13.0, and the new methods could not be
resolved, resulting in build failures.

[0]: https://developer.android.com/reference/android/view/View#requireViewById(int)
[1]: https://docs.microsoft.com/en-us/dotnet/api/android.views.view.requireviewbyid?view=xamarin-android-sdk-12
[2]: https://docs.microsoft.com/en-us/dotnet/api/android.views.view.findviewbyid?view=xamarin-android-sdk-12#android-views-view-findviewbyid-1(system-int32)

Author: jonathanpeppers

src/Mono.Android/Android.App/Activity.cs

@@ -12,6 +12,19 @@ partial class Activity {
 			return this.FindViewById (id)!.JavaCast<T> ();
 		}
 
+#if NET7_0_OR_GREATER || (NET6_0_OR_GREATER && ANDROID_33) || ANDROID_34
+		// See: https://cs.android.com/android/platform/superproject/+/master:frameworks/base/core/java/android/app/Activity.java;l=3430
+		public T RequireViewById<T> (int id)
+			where T : Android.Views.View
+		{
+			var view = FindViewById<T> (id);
+			if (view == null) {
+				throw new Java.Lang.IllegalArgumentException ($"Parameter 'id' of value 0x{id:X} does not reference a View of type '{typeof (T)}' inside this Activity");
+			}
+			return view;
+		}
+#endif // NET7_0_OR_GREATER || (NET6_0_OR_GREATER && ANDROID_33) || ANDROID_34
+
 		public void StartActivityForResult (Type activityType, int requestCode)
 		{
 			var intent = new Android.Content.Intent (this, activityType);

src/Mono.Android/Android.Views/View.cs

@@ -29,6 +29,19 @@ public T FindViewById<T> (int id)
 			return this.FindViewById (id).JavaCast<T> ();
 		}
 
+#if NET7_0_OR_GREATER || (NET6_0_OR_GREATER && ANDROID_33) || ANDROID_34
+		// See: https://cs.android.com/android/platform/superproject/+/master:frameworks/base/core/java/android/view/View.java;l=25322
+		public T RequireViewById<T> (int id)
+			where T : Android.Views.View
+		{
+			var view = FindViewById<T> (id);
+			if (view == null) {
+				throw new Java.Lang.IllegalArgumentException ($"Parameter 'id' of value 0x{id:X} does not reference a View of type '{typeof (T)}' inside this View");
+			}
+			return view;
+		}
+#endif //NET7_0_OR_GREATER || (NET6_0_OR_GREATER && ANDROID_33) || ANDROID_34
+
 		public bool Post (Action action)
 		{
 			return Post (new Java.Lang.Thread.RunnableImplementor (action, true));